Network Working Group Y. Cai
Internet-Draft H. Ou
Intended status: Standards Track Alibaba Group
Expires: April 25, 2019 S. Vallepalli
M. Mishra
S. Venaas
Cisco Systems, Inc.
A. Green
British Telecom
October 22, 2018
PIM Designated Router Load Balancingdraft-ietf-pim-drlb-09
Abstract
On a multi-access network, one of the PIM routers is elected as a
Designated Router (DR). On the last hop LAN, the PIM DR is
responsible for tracking local multicast listeners and forwarding
traffic to these listeners if the group is operating in PIM-SM. This
document specifies a modification to the PIM-SM protocol that allows
more than one of these last hop routers to be selected, so that the
forwarding load can be distributed among these routers.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 25, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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operating in PIM-SM. On the last hop LAN, the PIM DR is responsible
for tracking local multicast listeners and forwarding to these
listeners if the group is operating in PIM-SM.
Consider the following last hop LAN in Figure 1:
(core networks)
| | |
| | |
R1 R2 R3
| | |
--(last hop LAN)--
|
|
(many receivers)
Figure 1: Last Hop LAN
Assume R1 is elected as the Designated Router. According to
[RFC7761], R1 will be responsible for forwarding traffic to that LAN
on behalf of any local members. In addition to keeping track of IGMP
and MLD membership reports, R1 is also responsible for initiating the
creation of source and/or shared trees towards the senders or the
RPs.
Forcing sole data plane forwarding responsibility on the PIM DR
uncovers a limitation in the protocol. In comparison, even though an
OSPF DR or an IS-IS DIS handles additional duties while running the
OSPF or IS-IS protocols, they are not required to be solely
responsible for forwarding packets for the network. On the other
hand, on a last hop LAN, only the PIM DR is asked to forward packets
while the other routers handle only control traffic (and perhaps drop
packets due to RPF failures). Hence the forwarding load of a last
hop LAN is concentrated on a single router.
This leads to several issues. One of the issues is that the
aggregated bandwidth will be limited to what R1 can handle towards
this particular interface. It is very common that the last hop LAN
consists of switches that run IGMP/MLD or PIM snooping. This allows
the forwarding of multicast packets to be restricted only to segments
leading to receivers who have indicated their interest in multicast
groups using either IGMP or MLD. The emergence of the switched
Ethernet allows the aggregated bandwidth to exceed, sometimes by a
large number, that of a single link. For example, let us modify
Figure 1 and introduce an Ethernet switch in Figure 2.
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(core networks)
| | |
| | |
R1 R2 R3
| | |
+=gi0===gi1===gi2=+
+ +
+ switch +
+ +
+=gi4===gi5===gi6=+
| | |
H1 H2 H3
Figure 2: Last Hop Network with Ethernet Switch
Let us assume that each individual link is a Gigabit Ethernet. Each
router, R1, R2 and R3, and the switch have enough forwarding capacity
to handle hundreds of Gigabits of data.
Let us further assume that each of the hosts requests 500 Mbps of
unique multicast data. This totals to 1.5 Gbps of data, which is
less than what each switch or the combined uplink bandwidth across
the routers can handle, even under failure of a single router.
On the other hand, the link between R1 and switch, via port gi0, can
only handle a throughput of 1Gbps. And if R1 is the only DR (the PIM
DR elected using the procedure defined by [RFC7761]) at least 500
Mbps worth of data will be lost because the only link that can be
used to draw the traffic from the routers to the switch is via gi0.
In other words, the entire network's throughput is limited by the
single connection between the PIM DR and the switch (or the last hop
LAN as in Figure 1).
Another important issue is related to failover. If R1 is the only
forwarder on the last hop router for a shared LAN, when R1 goes out
of service, multicast forwarding for the entire LAN has to be rebuilt
by the newly elected PIM DR. However, if there was a way that
allowed multiple routers to forward to the LAN for different groups,
failure of one of the routers would only lead to disruption to a
subset of the flows, therefore improving the overall resilience of
the network.
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There is a limitation in the hash algorithm used in this document,
but this document provides the option to have different and more
consistent hash algorithms in the future.
This document specifies a modification to the PIM-SM protocol that
allows more than one of these routers, called Group Designated
Routers (GDR) to be selected so that the forwarding load can be
distributed among a number of routers.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
With respect to PIM, this document follows the terminology that has
been defined in [RFC7761].
This document also introduces the following new acronyms:
o GDR: GDR stands for "Group Designated Router". For each multicast
flow, either a (*,G) for ASM, or an (S,G) for SSM, a hash
algorithm (described below) is used to select one of the routers
as a GDR. The GDR is responsible for initiating the forwarding
tree building process for the corresponding multicast flow.
o GDR Candidate: a last hop router that has the potential to become
a GDR. A GDR Candidate must have the same DR priority and must
run the same GDR election hash algorithm as the DR router. It
must send and process new PIM Hello Options as defined in this
document. There might be more than one GDR Candidate on a LAN,
but only one can become GDR for a specific multicast flow.
3. Applicability
The extension specified in this document applies to PIM-SM last hop
routers only. It does not alter the behavior of a PIM DR on the
first hop network. This is because the source tree is built using
the IP address of the sender, not the IP address of the PIM DR that
sends the registers towards the RP. The load balancing between first
hop routers can be achieved naturally if an IGP provides equal cost
multiple paths (which it usually does in practice). Also
distributing the load to do registering does not justify the
additional complexity required to support it.
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Internet-Draft PIM Designated Router Load Balancing October 20184. Functional Overview
In the PIM DR election as defined in [RFC7761], when multiple last
hop routers are connected to a multi-access LAN (for example, an
Ethernet), one of them is elected to act as PIM DR. The PIM DR is
responsible for sending local Join/Prune messages towards the RP or
source. In order to elect the PIM DR, each PIM router on the LAN
examines the received PIM Hello messages and compares its own DR
priority and IP address with those of its neighbors. The router with
the highest DR priority is the PIM DR. If there are multiple such
routers, their IP addresses are used as the tie-breaker, as described
in [RFC7761].
In order to share forwarding load among last hop routers, besides the
normal PIM DR election, the GDR is also elected on the last hop
multi-access LAN. There is only one PIM DR on the multi-access LAN,
but there might be multiple GDR Candidates.
For each multicast flow, that is, (*,G) for ASM and (S,G) for SSM, a
hash algorithm is used to select one of the routers to be the GDR. A
new DR Load Balancing Capability (DRLBC) PIM Hello Option, which
contains hash algorithm type, is announced by routers on interfaces
where this specification is enabled. Last hop routers with the new
DRLBC Option advertised in its Hello, and using the same GDR election
hash algorithm and the same DR priority as the PIM DR, are considered
as GDR Candidates.
Hash Masks are defined for Source, Group and RP separately, in order
to handle PIM ASM/SSM. The masks, as well as a sorted list of GDR
Candidate Addresses, are announced by the DR in a new DR Load
Balancing GDR (DRLBGDR) PIM Hello Option.
A hash algorithm based on the announced Source, Group, or RP masks
allows one GDR to be assigned to a corresponding multicast state.
And that GDR is responsible for initiating the creation of the
multicast forwarding tree for multicast traffic.
4.1. GDR Candidates
GDR is the new concept introduced by this specification. GDR
Candidates are routers eligible for GDR election on the LAN. To
become a GDR Candidate, a router MUST support this specification,
have the same DR priority and run the same GDR election hash
algorithm as the DR on the LAN.
For example, assume there are 4 routers on the LAN: R1, R2, R3 and
R4, which all support this specification. R1, R2 and R3 have the
same DR priority while R4's DR priority is less preferred. In this
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example, R4 will not be eligible for GDR election, because R4 will
not become a PIM DR unless all of R1, R2 and R3 go out of service.
Furthermore, assume router R1 wins the PIM DR election, R1 and R2 run
the same hash algorithm for GDR election, while R3 runs a different
one. In this case, only R1 and R2 will be eligible for GDR election,
while R3 will not.
As a DR, R1 will include its own Load Balancing Hash Masks and the
identity of R1 and R2 (the GDR Candidates) in its DRLBGDR Hello
Option.
4.2. Hash Mask and Hash Algorithm
A Hash Mask is used to extract a number of bits from the
corresponding IP address field (32 for v4, 128 for v6) and calculate
a hash value. A hash value is used to select a GDR from GDR
Candidates advertised by PIM DR. For example, 0.0.255.0 defines a
Hash Mask for an IPv4 address that masks the first, the second, and
the fourth octets.
There are three Hash Masks defined:
o RP Hash Mask
o Source Hash Mask
o Group Hash Mask
The hash masks need to be configured on the PIM routers that can
potentially become a PIM DR, unless the implementation provides
default Hash Mask values. An implementation SHOULD provide masks
with default values 255.255.255.255 (IPv4) and
FFFF:FFFF:FFFF:FFFF:FFFFF:FFFF:FFFF:FFFF (IPv6).
o If the group is in ASM mode and the RP Hash Mask announced by the
PIM DR is not 0, calculate the value of hashvalue_RP [Section 4.3]
to determine GDR.
o If the group is in ASM mode and the RP Hash Mask announced by the
PIM DR is 0, obtain the value of hashvalue_Group [Section 4.3 ] to
determine GDR.
o If the group is in SSM mode, use hashvalue_SG [Section 4.3] to
determine GDR.
A simple Modulo hash algorithm is defined in this document. However,
to allow another hash algorithms to be used, a 1-octet "Hash
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Algorithm Type" field is included in DRLBC Hello Option to specify
the hash algorithm used by a last hop router.
If different hash algorithm types are advertised among last hop
routers, only last hop routers running the same hash algorithm as the
DR (and having the same DR priority as the DR) are eligible for GDR
election.
4.3. Modulo Hash Algorithm
The Modulo hash algorithm is discussed here with a detailed
description on hashvalue_RP. The same algorithm is described in
brief for hashvalue_Group using the group address instead of the RP
address for an ASM group with zero RP_hashmask, and also with
hashvalue_SG for a the source address of an (S,G), instead of the RP
address,
o For ASM groups, with a non-zero RP_Hash Mask, hash value is
calculated as:
hashvalue_RP = (((RP_address & RP_hashmask) >> N) & 0xFFFF) % M
RP_address is the address of the RP defined for the group. N
is the number of zeroes, counted from the least significant bit
of the RP_hashmask. M is the number of GDR Candidates.
For example, Router X with IPv4 address 203.0.113.1 receives a
DRLBGDR Hello Option from the DR, which announces RP Hash Mask
0.0.255.0 and a list of GDR Candidates, sorted by IP addresses
from high to low: 203.0.113.3, 203.0.113.2 and 203.0.113.1.
The ordinal number assigned to those addresses would be:
0 for 203.0.113.3; 1 for 203.0.113.2; 2 for 203.0.113.1 (Router
X)
Assume there are 2 RPs: RP1 192.0.2.1 for Group1 and RP2
198.51.100.2 for Group2. Following the modulo hash algorithm:
N is 8 for 0.0.255.0, and M is 3 for the total number of GDR
Candidates. The hashvalue_RP for RP1 192.0.2.1 is:
(((192.0.2.1 & 0.0.255.0) >> 8) & 0xFFFF % 3) = 2 % 3 = 2
matches the ordinal number assigned to Router X. Router X will
be the GDR for Group1, which uses 192.0.2.1 as the RP.
The hashvalue_RP for RP2 198.51.100.2 is:
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(((198.51.100.2 & 0.0.255.0) >> 8) & 0xFFFF % 3) = 100 % 3 = 1
which is different from Router X's ordinal number(2) hence,
Router X will not be GDR for Group2.
o If RP_hashmask is 0, a hash value for an ASM group is calculated
using the Group Hash Mask:
hashvalue_Group = (((Group_address & Group_hashmask) >> N) &
0xFFFF) % M
Compare hashvalue_Group with Ordinal number assigned to Router
X, to decide if Router X is the GDR.
o For SSM groups, a hash value is calculated using both the Source
and Group Hash Mask:
hashvalue_SG = ((((Source_address & Source_hashmask) >> N_S) &
0xFFFF) ^ (((Group_address & Group_hashmask) >> N_G) & 0xFFFF))
% M
4.3.1. Limitations
The Modulo Hash Algorithm has poor failover characteristics when a
shared LAN has more than two GDRs. In the case of more than two GDRs
on a LAN, when one GDR fails, all of the groups may be reassigned to
a new GDR, even if they were not assigned to the failed GDR.
However, many deployments use only two routers on a shared LAN for
redundancy purposes. Future work may define new hash algorithms
where only groups assigned to the failed GDR get reassigned.
4.4. PIM Hello Options
When a last hop PIM router sends a PIM Hello for an interface with
this specification enabled, it includes a new option, called "Load
Balancing Capability (DRLBC)".
Besides this DRLBC Hello Option, the elected PIM DR also includes a
new "DR Load Balancing GDR (DRLBGDR) Hello Option". The DRLBGDR
Hello Option consists of three Hash Masks as defined above and also a
sorted list of GDR Candidate addresses on the last hop LAN.
The elected PIM DR uses DRLBC Hello Option advertised by all routers
on the last hop LAN to compose the DRLBGDR Option. The GDR
Candidates use the DRLBGDR Hello Option advertised by the PIM DR to
calculate the hash value.
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Group Mask (32/128 bits): Mask
Source Mask (32/128 bits): Mask
RP Mask (32/128 bits): Mask
All masks MUST be in the same address family as the Hello IP
header.
GDR Address (32/128 bits): Address(es) of GDR Candidate(s)
All addresses must be in the same address family as the Hello
IP header. The addresses are sorted in descending order. The
order is converted to the ordinal number associated with each
GDR candidate in hash value calculation. For example, if
addresses advertised are R3, R2, R1, the ordinal number
assigned to R3 is 0, to R2 is 1 and to R1 is 2.
If the "Interface ID" option, as specified in [RFC6395], is
present in a GDR Candidate's PIM Hello message, and the "Router
ID" portion is non-zero:
+ For IPv4, the "GDR Candidate Address" will be set directly
to the "Router ID".
+ For IPv6, the "GDR Candidate Address" will be set to the
IPv4-IPv6 translated address of the "Router ID", as
described in [RFC4291], that is the "Router-ID" is appended
to the prefix of 96 bits of zeroes.
If the "Interface ID" option is not present in a GDR
Candidate's PIM Hello message, or if the "Interface ID" option
is present but the "Router ID" field is zero, the "GDR
Candidate Address" will be the IPv4 or IPv6 source address of
the PIM Hello message.
This DRLBGDR Hello Option MUST only be advertised by the
elected PIM DR.
6. Protocol Specification6.1. PIM DR Operation
The DR election process is still the same as defined in [RFC7761]. A
DR that has this specification enabled on an interface advertises the
new DRLBGDR Hello Option, which contains mask values from user
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configuration, followed by a sorted list of GDR Candidate Addresses,
from the highest value to the lowest value. Moreover, same as non-DR
routers, the DR also advertises DRLBC Hello Option to indicate its
capability of supporting this specification and the type of its GDR
election hash algorithm.
If a PIM DR receives a PIM Hello with the DRLBGDR Option, the PIM DR
SHOULD ignore the TLV.
If a PIM DR receives a neighbor DRLBC Hello Option, which contains
the same hash algorithm type as the DR, and the neighbor has the same
DR priority as the DR, PIM DR SHOULD consider the neighbor as a GDR
Candidate and insert the GDR Candidate's Address into the sorted list
of the DRLBGDR Option. However, the DR MAY have policies limiting
which GDR Candidates, or the number of GDR Candidates to include.
6.2. PIM GDR Candidate Operation
When an IGMP/MLD report is received, without this specification, only
the PIM DR will handle the join and potentially run into the issues
described earlier. Using this specification, a hash algorithm is
used by the GDR Candidates to determine which router is going to be
responsible for building forwarding trees on behalf of the host.
If this specification is enabled on an interface, the router MUST
include the DRLBC Hello Option in its PIM Hello on the interface.
Note that the presence of the DRLBC Option in PIM Hello does not
guarantee that this router would be considered as a GDR candidate.
Once DR election is done, the DRLBGDR Hello Option would be received
from the current PIM DR on the link which would contain a list of
GDRs selected by the PIM DR.
A router only acts as a GDR candidate if it is included in the GDR
list of the DRLBGDR Hello Option.
A GDR Candidate may receive a DRLBGDR Hello Option from the PIM DR
with different Hash Masks from those the candidate was configured
with. The GDR Candidate MUST use the Hash Masks advertised by the
PIM DR to calculate the hash value.
A GDR Candidate MUST ignore the DRLBGDR Hello Option if it is
received from a PIM router which is not the DR.
If the PIM DR does not support this specification, GDR election will
not take place, and only the PIM DR joins the multicast tree.
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Internet-Draft PIM Designated Router Load Balancing October 20186.2.1. Router Receives New DRLBGDR
The first time a router receives a DRLBGDR option from the PIM DR, it
MUST process the option and check if it is in the GDR list.
1. If a router is not listed as a GDR candidate in DRLBGDR, no
action is needed.
2. If a router is listed as a GDR candidate in DRLBGDR, then it MUST
process each of the groups, or source and group pairs if SSM, in
the IGMP/MLD reports. The masks are announced in the PIM Hello
by the DR in the DRLBGDR Hello Option. For each group in the
reports that is in ASM mode, and each source and group pair if
the group is in SSM mode, it (PIM Router) needs to run the hash
algorithm (described in section 4.3) based on the announced
Source, Group or RP masks to determine if it is the GDR for
specified group, or source and group pair. If the hash result is
to be the GDR for the multicast flow, it does build the multicast
forwarding tree. If it is not the GDR for the multicast flow, no
action is needed.
6.2.2. Router Receives Updated DRLBGDR
If a router (GDR or non GDR) receives an unchanged DRLBGDR from the
current PIM DR, no action is needed.
If a router (GDR or non GDR) receives a new or modified DRLBGDR from
the current PIM DR, it requires processing as described below:
1. If it was included in the previous GDR list, and still is
included in the new GDR list: It needs to process each of the
groups, or source and group pairs if the group is in SSM mode,
and run the hash algorithm to check if it is still the GDR for
the given group, or source and group pair if SSM.
If it was the GDR for a group, or source and group pair if
SSM, and the new hash result chose it as the GDR, then no
processing is required.
If it was the GDR for a group, or source and group pair if
SSM, earlier and now it is no longer the GDR, then it sets its
assert metric for the multicast flow to be
(PIM_ASSERT_INFINITY - 1), as explained in Section 6.3.
If it was not the GDR for a group, or source and group pair if
SSM, earlier, and the new hash does not make it GDR, then no
processing is required.
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If it was not the GDR for an earlier group, or source and
group pair if SSM, and now becomes the GDR, it starts building
multicast forwarding tree for this flow.
2. If it was included in the previous GDR list, but is not included
in the new GDR list: It needs to process each of the groups, or
source and group pairs if the group is in SSM mode.
If it was the GDR for a group, or source and group pair if
SSM, it sets its assert metric for the multicast flow to be
(PIM_ASSERT_INFINITY - 1), as explained in Section 6.3.
If it was not the GDR, then no processing is required.
3. If it was not included in the previous GDR list, but is included
in the new GDR list, the router MUST run the hash algorithm for
each of the groups, source and group pairs if SSM.
If it is not the GDR for a group, or source and group pair if
SSM, no processing is required.
If it is hashed as the GDR, it needs to build a multicast
forwarding tree.
6.3. PIM Assert Modification
It is possible that the identity of the GDR might change in the
middle of an active flow. Examples when this could happen include:
When a new PIM router comes up
When a GDR restarts
When the GDR changes, existing traffic might be disrupted.
Duplicates or packet loss might be observed. To illustrate the case,
consider the following scenario where there are two flows G1 and G2.
R1 is the GDR for G1, and R2 is the GDR for G2. When R3 comes up
online, it is possible that R3 becomes GDR for both G1 and G2, hence
R3 starts to build the forwarding tree for G1 and G2. If R1 and R2
stop forwarding before R3 completes the process, packet loss might
occur. On the other hand, if R1 and R2 continue forwarding while R3
is building the forwarding trees, duplicates might occur.
This is not a typical deployment scenario but might still happen.
Here we describe a mechanism to minimize the impact. We essentially
want to minimize packet loss. Therefore, we would allow a small
amount of duplicates and depend on PIM Assert to minimize the
duplication.
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When the role of GDR changes as above, instead of immediately
stopping forwarding, R1 and R2 continue forwarding to G1 and G2
respectively, while, at the same time, R3 build forwarding trees for
G1 and G2. This will lead to PIM Asserts.
With the introduction of GDR, the following modification to the
Assert packet MUST be done: if a router enables this specification on
its downstream interface, but it is not a GDR (before network event
it was GDR), it would adjust its Assert metric to
(PIM_ASSERT_INFINITY - 1).
Using the above example, for G1, assume R1 and R3 agree on the new
GDR, which is R3. R1 will set its Assert metric as
(PIM_ASSERT_INFINITY - 1). That will make R3, which has normal
metric in its Assert as the Assert winner.
For G2, assume it takes a slightly longer time for R2 to find out
that R3 is the new GDR and still considers itself being the GDR while
R3 already has assumed the role of GDR. Since both R2 and R3 think
they are GDRs, they further compare their metric and IP addresses.
If R3 has the better routing metric, or the same metric but a better
tie-breaker, the result will be consistent during GDR selection. If
unfortunately, R2 has the better metric or the same metric but a
better tie-breaker, R2 will become the Assert winner and continues to
forward traffic. This will continue until:
The next PIM Hello Option from DR selects R3 as the GDR. R3 will
then build the forwarding tree and send an Assert.
The process continues until R2 agrees to the selection of R3 as the
GDR, and sets its own Assert metric to (PIM_ASSERT_INFINITY - 1),
which will make R3 the Assert winner. During the process, we will
see intermittent duplication of traffic but packet loss will be
minimized. In the unlikely case that R2 never relinquishes its role
as GDR (while every other router thinks otherwise), the proposed
mechanism also helps to keep the duplication to a minimum until
manual intervention takes place to remedy the situation.
7. Compatibility
In the case of a hybrid Ethernet shared LAN (where some PIM routers
enable the specification defined in this document, and some do not)
o If a router which does not support this specification becomes the
DR on the LAN, then it is the only router acting as a DR, and
there will be no load-balancing.
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o If a router which does not support this specification becomes a
non-DR on link, then it acts as non-DR defined in [RFC7761], and
it will not take part in any load-balancing.
8. Manageability Considerations
Only the routers announcing the same Hash Algorithm as the DR would
be considered as GDR candidates. Network administrators need to make
sure that the desired set of routers announce the same algorithm.
Migration between different algorithm types is not considered in this
document.
9. IANA Considerations
IANA has temporarily assigned type 34 for the PIM DR Load Balancing
Capability (DRLBC) Hello Option, and type 35 for the PIM DR Load
Balancing GDR (DRLBGDR) Hello Option. IANA is requested to make
these assignments permanent when this document is published as an
RFC. The string TBD should be replaced by the assigned values
accordingly. This document requests IANA to create a DRLB hash type
registry. This should be placed in the "Protocol Independent
Multicast (PIM)" branch of the tree.
9.1. Initial registry
The initial content of the registry should be as follows.
Type Name Reference
------ ---------------------------------------- --------------------
0 Hash algorithm modulo This document
1-255 Unassigned
9.2. Assignment of new message types
Assignment of new message types is done according to the "IETF
Review" model, see [RFC5226].
10. Security Considerations
Security of the new DR Load Balancing PIM Hello Options is only
guaranteed by the security of PIM Hello messages, so the security
considerations for PIM Hello messages as described in PIM-SM
[RFC7761] apply here.
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